Not applicable.
1. Field of the Invention
This invention relates to shelled Pdxe2x80x94Au catalyst of particular characteristics, and methods for their production, which are effective for catalyzing the vapor phase reaction of an alkene (such as ethylene) with an alkanoic acid (such as acetic acid) and oxygen to produce an alkenyl alkanoate (such as vinyl acetate) at high values for space-time yield, specific activity, and with a high selectivity for conversion of the alkene to the alkenyl alkanoate (such as ethylene to vinyl acetate).
2. Description of the Related Art
Vinyl acetate (VA) is a commodity chemical in high demand as a monomer for production of poly(vinyl acetate). This important polymer, and its derivatives, finds extensive uses as adhesives, paints and other coatings, films and laminating materials. Many techniques have been reported in the prior art for the production of VA. A chief technique is a catalyzed gas phase reaction of ethylene with acetic acid and oxygen. Today a type of catalyst widely use for this reaction is a surface shell impregnated catalyst of a type as described in U.S. Pat. No. 4,048,096 by T. C. Bissot.
Bissot""s U.S. Pat. No. 4,048,096 discloses a catalyst having a specific activity of at least about 83 grams of vinyl acetate per gram of precious metal (Pd+Au) per hour measured at 150xc2x0 C. and a reaction pressure of 120 psig. The catalyst consists of: (1) catalyst support particles having a particle diameter of from about 3 to about 7 mm and a pore volume of from about 0.2 to about 1.5 ml/g, (2) palladium and gold distributed in a surface layer of the catalyst support extending less than about 0.5 mm into the support, the palladium being present in an amount of from about 1.5 to about 5.0 grams per liter of catalyst, and the gold being present in an amount of from about 0.5 to about 2.25 grams per liter of catalyst, and (3) from about 5 to about 60 grams per liter of catalyst of an alkali metal acetate. Palladium is the active catalyst metal and the gold is a catalyst promoter.
The Bissot ""096 patent process for catalyst preparation comprises: (1) impregnating the catalyst support with an aqueous solution of water-soluble palladium and gold compounds, (2) precipitating water-insoluble palladium and gold compounds on the catalyst support surface by contacting the impregnated catalyst support with a solution of compounds (preferably sodium metasilicate) capable of reacting with the water-soluble palladium and gold compounds to form water-insoluble palladium and gold compounds, (3) converting the water-insoluble palladium and gold compounds into palladium and gold metal on the support surface by treatment with a reducing agent, (4) washing the catalyst with water, (5) drying the catalyst, (6) impregnating the catalyst with an alkali metal acetate promoter (e.g., a potassium promoter), and (7) drying the catalyst.
The improvement disclosed in Bissot ""096, as compared to prior Pdxe2x80x94Au supported catalysts, involves distributing the catalyst loading of palladium and gold as a surface layer on the catalyst support which is less than about 0.5 millimeter into the support from its surface. The impregnating step is carried out with an aqueous solution of palladium and gold compounds and the total volume of the solution is from about 95 to about 100% of the absorptive capacity of the catalyst support. The precipitating step in Bissot is carried out by soaking the wet catalyst support with a solution of an alkali metal silicate, the amount of alkali silicate being such that, after the alkali metal silicate solution has been in contact with the catalyst support for about 12 to 24 hours, the pH of said solution is from about 6.5 to about 9.5. In all examples of Bissot the reduction of the precipitated compounds to Pd and Au metals is accomplished by reaction with a hydrazine solution.
As is apparent from a reading of the Bissot patent, a major concern in this art of vinyl acetate (VA) production has always been to improve the space-time yield (STY) and also the specific activity (SA) of the catalysts. Since the description of this shell type of catalyst by Bissot others have attempted to improve the catalyst in respect to its space-time yield, specific activity, and/or its selectivity
In U.S. Pat. Nos. 5,179,056; 5,189,004; and 5,342,987 by W. J. Barley it is reported that a shell impregnated catalysts of the Bissot type is improved in respect to its STY if it is essentially free of sodium; such as if it prepared from ingredients that are essentially free of sodium as per the ""056 patent, or if its sodium content is removed by washing with water or an aqueous solution of a potassium promoter as in the ""004 patent, or by washing the catalyst at an intermediate stage of its production with an ion exchange solution as in the ""987 patent. In all of the above patents the exemplified catalyst are reduced with hydrazine solutions. U.S. Pat. No. 5,693,586 reports that a shell impregnated catalysts of the Bissot type which are made from reagents that are all potassium salt compounds are of an improved carbon dioxide selectivity. In this patent all example catalyst are reduced with ethylene at a temperature of 150xc2x0 C.
Barley et al. in U.S. Pat. No. 5,274,181 reports that a shell impregnated catalysts of the Bissot type is improved in respect to its STY if it is prepared to have, at a Pd loading of 2.5 xcexcL (0.33 wt %) to 6.1 g/L (1.05 wt %), a weight ratio of Au to Pd in the range of 0.6 to 1.25. All catalyst examples of this patent are reduced by reaction with a hydrazine solution.
U.S. Pat. No. 5,567,839 reports that a shell impregnated catalysts of the Bissot type is improved in respect to its STY if a barium salt rather than a sodium silicate is use to precipitate the Pd and Au compounds into the shell. All catalyst examples of this patent are reduced by reaction with a hydrazine solution.
The selectivity of a palladium-gold catalyst in vinyl acetate synthesis also is influenced by the extent and uniformity of the palladium metal and gold metal distribution on the exterior and/or interior surfaces of a porous catalyst support substrate, such as carbon dioxide selectivity and oxygen conversion in an ethylene, acetic acid and oxygen vapor phase reaction.
Attempts to provide a uniform distribution of the palladium and gold metals on the catalyst support has involved manipulation of the catalyst preparation steps and/or by using support substrates having various specified pore dimensions. Particularly useful improvements in preparing highly active catalysts for vinyl acetate production are disclosed in U.S. Pat. No. 5,314,858 and U.S. Pat. No. 5,332,710. These references describe process embodiments for improving palladium and gold distribution on a support by manipulating the precipitation step in which the water-soluble precious metal compounds are fixed to the support surface as water-insoluble compounds. In U.S. Pat. No. 5,314,858, fixing precious metals on the support is achieved utilizing two separate precipitation stages to avoid using large excesses of fixing agent. U.S. Pat. No. 5,332,710 describes fixing the precious metals by physically rotating an impregnated catalyst support while the impregnated support is immersed in a reaction solution at least during the initial precipitation period. The rotation immersion procedure yields catalysts in which the metals precipitated on the carrier are said to be more evenly distributed in a thin shell on the support surface. All catalyst examples of these patents are reduced with ethylene at a temperature of 150xc2x0 C.
U.S. Pat. No. 6,420,308 by A. K. Khanmamedova (Khanmamedova""308) showed that the sequence of steps is critical. In Bissot""096 the catalyst was washed after reduction whereas in Khanmamedova""308 the catalyst was washed before reduction and obtained higher space time yield, and higher vinyl acetate selectivity.
Despite such improvements as have been made there is a continuing interest in the development of catalyst compositions that exhibit an even further improved combination of properties for the production of vinyl acetate.
This invention relates to a shell impregnated catalyst of Pdxe2x80x94Au, and methods for their production, which are effective for catalyzing the vapor phase reaction of ethylene with acetic acid and oxygen to produce vinyl acetate at high values for space-time yield, specific activity, and with a high selectivity for conversion of ethylene to vinyl acetate. This invention uses the same sequence of steps as in Khanmamedova but adds certain improvements:
1) palladium to gold weight ratio in the range from 2:8 to 8:2,
2) precipitating palladium and gold compounds to 70 to 100% of the total pore volume of the support particles,
3) a 1.1 to 2.2 molar excess of precipitating agent,
4) contact time with the precipitating agent for three hours up to seventy-two hours,
5) contact with the precipitating agent in light protected environment,
6) using a powdered precipitating agent,
7) washing the precipitated support with water at a temperature of from 50 to 80xc2x0 C.,
8) reducing the catalyst precursor in nitrogen,
9) impregnating with potassium alkanoate in a solution of 5-12 wt. %,
10) final drying of the catalyst in nitrogen at 95xc2x0 C. to 150xc2x0 C. for one hour to twenty-four hours.
The shell impregnated catalyst of Pdxe2x80x94Au are produced on a silica support to have a Pd loading of 1.8 to about 7.2 g/L of catalyst and a Pd:Au weight ratio of 8:2 to 2:8 by impregnating the support with aqueous solutions of palladium and gold salts or acids, which preferably are high purity potassium tetrachlorpalladate (99.99%) and hydrogentetrachloraurate trihydrate (99.998%), and thereafter precipitating or xe2x80x9cfixingxe2x80x9d, preferably in a light protected environment, water insoluble compounds of Pd and Au on the support surface by reaction for three to seventy-two hours of the impregnated support with solutions or powders of alkali metal metasilicates or alkali metal hydroxides or mixtures thereof as precipitating or xe2x80x9cfixingxe2x80x9d agents, preferably with a sodium metasilicate solution being used as a precipitating agent, to a volume corresponding to 70% to 100% of the total pore volume of the support particle in a quantity that exceeds the theoretical amount required to neutralize the Pd and Au salts to a 1.1 to 2.2 molar excess. The excess of fixing agent also depends on volume of fixing solution and acidity of support.
The impregnated support is then washed with deionized water at a temperature of 50xc2x0 C. to 80xc2x0 C. until the final decant is negative to a silver nitrate test, after which it is dried for water removal. The dried support with its surface precipitated compounds of Pd and Au is then reacted with ethylene, nitrogen or hydrogen at a temperature greater than 150xc2x0 C. and for ethylene up to 310xc2x0 C., for nitrogen up to 500xc2x0 C. and for hydrogen up to 299xc2x0 C., preferably for 10 minutes to one hour at a temperature of from 250xc2x0 C. to 325xc2x0 C. for ethylene, more preferably 250xc2x0 C. to 310xc2x0 C., from 300xc2x0 to 450xc2x0 C. for nitrogen and from 250xc2x0 C. to 299xc2x0 C. for hydrogen, until substantially all of its content of Pd and Au are reduced to their free metal state, after which the support is impregnated with potassium acetate to an extent of 5 to 12 weight percent of the weight of the reduced catalyst. Thereafter the catalyst is dried in air or in nitrogen at 90-150xc2x0 C. for one to twenty four hours, preferably in nitrogen at 120xc2x0 C. for one hour.
A catalyst as described above has a space-time yield (STY) and specific activity (SA) about 20-30% greater than an otherwise identical catalyst composition that is reduced with ethylene or hydrogen at 150xc2x0 C. In a temperature range of 140xc2x0 C. to 160xc2x0 C. at a gas hourly space velocity of 4500/hr the catalyst will exhibit a vinyl acetate selectivity of 90% or greater when operated under reaction conditions that result in a STY of at least 600 gVA/L catalyst/hr. Further, such catalysts have a long operational life.
This invention comprises a catalyst for the promotion of a gas phase reaction of an alkene, an alkanoic acid, and an oxygen-containing gas to produce an alkenyl alkanoate. The catalyst is particularly desirable for the gas phase reaction of an ethylene, an acetic acid, and an oxygen-containing gas to produce vinyl acetate.
In the catalyzed gas phase reaction process, ethylene reacts exothermically with acetic acid and oxygen in the vapor phase over a heterogeneous Pdxe2x80x94Au shelled catalysts, giving vinyl acetate and water:
CH2xe2x95x90CH2+CH3CO2H+xc2xdO2xe2x86x92CH3CO2CHxe2x95x90CH2+H2O, xcex94H=xe2x88x92178 kJ/mol. 
The vinyl acetate reaction process may typically operate at 140-180xc2x0 C., 5-10 atmospheres (atm), and a gas hourly space velocity (GHSV) of xcx9c4500 hxe2x88x92. This will give 8-10% ethylene and 15-40% acetic acid conversion. Oxygen conversion can be up to 90%, and the yields are up to 99% and 94% based on acetic acid and ethylene, respectively.
Reaction temperatures may be between 140xc2x0 C. and 200xc2x0 C. Generally the preferred reaction temperature range is 140xc2x0 C. to 180xc2x0 C. with 140-160xc2x0 C. being most preferred. At temperatures below 140xc2x0 C., the reaction rate is low and it is difficult to keep the acetic acid in the vapor phase. Above 180xc2x0 C., for a given catalyst, more and more of the ethylene and acetic acid feeds are converted to by products. The principal by product is carbon dioxide. Generally, the other by-products, acetaldehyde, and ethyl acetate are formed at about 1% or less.
Reaction pressures are between 70-200 psig. Typically, the pressure used in commercial plants is 100-150 psig. Higher pressures make it difficult to keep the acetic acid in the vapor phase whereas pressures lower than 70 psig too greatly reduce the STY of the reaction.
The total volume of reaction gases as a gas hourly space velocity (GHSV) is about 3000-5000 STP liter/liter of catalyst per hour. Higher GHSV values result in higher STY and SA values without significantly lowering the selective values for production of vinyl acetate. Therefore, higher GHSV values, such 4500, are preferred. The composition of the reaction gases in volume % is in the range of ethylene, 27-60%; inerts 15-55%; acetic acid 12-17% and oxygen 6-8%. The reaction is operated with a large excess of ethylene and acetic acid. The main reason for doing so is to avoid formation of potentially flammable/explosive mixtures. Oxygen levels above about 9% are not used in order to avoid explosive mixtures. The preferred ranges, respectively are ethylene 50-60%, inerts 20-50%, acetic acid 12-15%, and oxygen 6-7%. Commercially, oxygen is often used in place of air and the percentage of ethylene in the feed is raised.
The support particles used in the process of producing catalyst of this invention are solid particulate materials that are capable of being impregnated with palladium, gold and a potassium promoter and that are inert under the conditions used to produce alkenyl alkanoates, such as vinyl acetate. Illustrative of such support particles are particulate silica, alumina, and silica-aluminas. Silica is the preferred support. The support preferably has a surface area from 100 to 800 square meters per gram. Silica beads of an average diameter of 5 to 6 mm, a surface area of 150 to 200 square meters per gram and a pore volume of 0.6 to 0.7 ml/g, such as xe2x80x9cKA-160xe2x80x9d sold by Sud Chemie AG, is an example of a most preferred support material.
The aqueous solutions of water-soluble palladium and gold compounds used in the process of this invention may include aqueous solutions of any suitable palladium or gold compound such as palladium (II) chloride, alkali earth metal tetrachloropalladium (II), palladium (II) nitrate, palladium (II) sulfate, gold (III) chloride or auric (III) acid (HAuCl4). However, compounds containing sodium are less preferred and the preferred compounds are potassium tetrachlorpalladate and hydrogentetrachloraurate. Then, for obtaining a high value for the space-time yield (STY) and specific activity (SA) of the catalyst it is preferred to utilize these preferred compounds in their high purity form, meaning 99.9+%purity, preferably 99.99%. Hence, it is preferred to use a potassium tetrachloropalladium of 99.99% purity and hydrogentetrachloraurate of 99.998% purity.
The quantity of Pd and Au compounds to be employed is such as to provide in the final catalyst a Pd loading of from about 1.8 g/L to about 7.2 g/L and a Au loading that places Au in the catalyst in a weight ratio to Pd in the Pd:Au range from 8:2 to 2:8, preferably 7:3 to 4:6, more preferably 6:4 to 5:5. Preferably the quantity of Pd loaded in the catalyst is such to provide the catalyst with a specific activity of greater than 200 g VA/g Pd/hr when operated under reaction conditions of 120 psig and within a temperature range of about 140xc2x0 C. to about 160xc2x0 C. that provide a STY of at least about 600 gVA/L cat/hr. The lower is the Pd loading that can be used to obtain the requisite STY values the higher will be the selectivity of conversion to VA, hence Pd loadings in a range of about 3.0 g/L to about 5.4 g/L are preferred.
The support is impregnated in a process designated as xe2x80x9crotation immersion to a point of incipient wetness.xe2x80x9d The volume of the impregnation solution preferably corresponds to from 70 to 100% (more preferably from 85 to 95%, most preferably about 90%) of the pore volume of the support. In this process, the catalyst support is immersed in the Pdxe2x80x94Au impregnation solution and tumbled or rotated therein during the initial stages of the impregnation of the support with the soluble precious metal compounds. The rotation or tumbling of the supports in the solution should proceed until all of the solution is absorbed which will depend on the volume of Pdxe2x80x94Au impregnating solution. Typically, for laboratory quantities, rotation is for at least 3 minutes and, preferably, for 3-15 minutes and more preferably 3-5 minutes. Excessive rotation can cause evaporation, loss of water and drying which can result in non-uniform distribution of palladium and gold, especially when the volume of the impregnation solution corresponds to 90% or less of the pore volume of the support.
Any type of rotation or tumbling equipment can be used as the exact apparatus utilized is not critical. However the extent of the rotating motion may be critical. The rotation should be fast enough so that all surfaces of the impregnated supports are evenly contacted with the impregnation solution as soon as possible. The rotation should not be so harsh that actual abrasion of the support particles takes place. Generally, the extent of rotation should be about 1 to 30 rpm and possibly even higher especially in the beginning of rotation depending upon the exact support utilized, the amount of support and the amount of precious metal to be impregnated into the support. The rpm to be used is variable and may also depend upon the apparatus utilized, the size and shape of the support, the type of support, and metal loading.
The precipitating agents used in the process of the present invention catalysts include sodium, lithium and potassium silicates, hydroxides and chlorides. It is preferred to use potassium chloride, sodium metasilicate, sodium hydroxide or potassium hydroxide as the precipitating agent. The precipitating agents are preferably employed in the form of aqueous solutions containing a 1.1 to 2.2 molar excess of the precipitating agents depending on support acidity and volume of used solution, preferably a 1.2 to 2.1 molar excess, more preferably a 1.5 molar excess. The volume of such solutions used is preferably just sufficient to cover the support particles. The impregnated support is immersed into the fixing solution and allowed to remain completely covered for three hours up to about 3 days (approximately 72 hours), preferably fifteen hours to 3 days, more preferably 1 day (approximately 24 hours) to 3 days, most preferably 3 days) at room temperature until a final pH value of 6.5-9.0 is attained. The exact quantity of alkali, time of fixing and final pH is dependent on the alkali type, the acidity of the support, and the quantities of precious metals used.
While the precipitating agent may be an aqueous solution, it may also be a powder used in a xe2x80x9cdry fixingxe2x80x9d technique to precipitate compounds of palladium and gold on the support surface. The powder may be added to the impregnated support beads or the impregnated support beads may be added to the powder. The solidxe2x80x94solid mixture may be rotated or agitated to uniformly mix the powder and the impregnated support beads in order to ensure that the precipitating agent is brought into contact with the water-soluble palladium and gold compounds so that water-insoluble palladium and gold compounds may be precipitated onto the support particles. The contact time may be the same as for the aqueous solution, i.e., three hours up to about 3 days (xcx9c72 hours), but preferably will be 12 to 36 hours, more preferably about 24 hours.
The reaction during the fixing step may be photosensitive. The source of gold, e.g., hydrogen tetrachloroaurate trihydrate, used in the catalyst synthesis may be sensitive to light and decompose with formation of separate phases. While the invention is not to be limited or restricted by theory, it is believed that the gold and palladium compounds are transformed to hydroxides of gold and palladium during the fixing step which, when reduced, result in palladium-gold interaction, with possible alloy formation. Gold hydroxides and palladium hydroxides separated on the surface of a support will be reduced with formation of inert gold metal and less active palladium metal. Exposure to light may decompose the gold and/or palladium compounds and may interfere with the formation of mixed hydroxides.
After fixation is completed the impregnated support beads are then removed from fixing solution and washed with deionized (D.I.) water at a temperature of 50 to 80xc2x0 C., preferably 50 to 60xc2x0 C. Further washing may then done in a batch or a continuous mode. Further washing at 50 to 80xc2x0 C., preferably 50 to 60xc2x0 C., should continue until the decant wash water content of chlorine ions is around 50 ppm, wherein the final decant gives a negative result to a silver nitrate test. Washing in water at a temperature above room temperature is believed not only to remove chlorine ions and sodium ions but also to remove amorphous silica particles from the pores.
After washing is complete the impregnated support beads are dried, such as at 90-150xc2x0 C. in a forced air oven.
The reducing agent used in the process of this invention is ethylene, nitrogen or hydrogen to which the dried impregnated support are exposed while at a temperature greater than 150xc2x0 C. and up to 310xc2x0 C. for ethylene, up to 500xc2x0 C. for nitrogen and up to 299xc2x0 C. for hydrogen, preferably of or greater than 200xc2x0 C. and more preferably to a temperature greater than 250xc2x0 C. such as a range of 275xc2x0 to 310xc2x0 C. for ethylene, 300xc2x0 to 450xc2x0 C. for nitrogen and 275xc2x0 C. to 299xc2x0 C. for hydrogen, and most preferably at 300xc2x0 C. for ethylene, 450xc2x0 C. for nitrogen and 299xc2x0 C. for hydrogen, for a time sufficient to complete the reduction of Pd and Au to their free metal state. Generally, the reduction is carried out for no longer than five hours, and preferably reduction in ethylene for less than one hour, preferably reduction in nitrogen for about one hour, and preferably reduction in hydrogen for about 10 to 30 minutes.
For purposes of their reduction the impregnated support beads may first be heated in a flow of gas, such as a nitrogen flow, from room temperature to 150xc2x0 C. The impregnated support may then be held in the gas flow at 150xc2x0 C. for 0.5-1 hour. Adsorbed water evolves during this heating period. The temperature may then be raised to 299-300xc2x0 C., preferably increasing at 20-30xc2x0/min. Optionally, the impregnated support beads may be maintained at 299-300xc2x0 C. for 10-30 minutes. Then the catalyst beads may be exposed to the reducing gas. If the reducing gas is ethylene or hydrogen, a volume of ethylene or hydrogen (1-5% by volume, preferably) may be introduced into the gas flow to form a reducing gas mixture.
For purpose of their reduction, dried impregnated beads may be directly placed into a heater at 300xc2x0 C. with an ethylene-nitrogen gas flow mixture or at 299xc2x0 C. with a hydrogen-nitrogen gas flow mixture or at 450xc2x0 C. with a nitrogen gas flow. Dried beads should not be exposed to moisture before reduction. The reducing gas flow has to be sufficient to provide complete reduction of the catalyst metals and may be varied in contact time range.
After about 10-15 minutes to about 5 hours of reduction in the reducing gas or reducing gas mixture, the beads may then be cooled back to room temperature in nitrogen. Shorter reduction times yield catalysts of higher STY values; hence shorter reduction times of from about 15 minutes to about 0.5 hour are preferred.
The potassium promoters used in the process of this invention for producing the catalysts may include potassium alkanoates and any potassium compound that is converted to a potassium alkanoate during the alkenyl alkanoate-forming reaction (i.e., the reaction of ethylene, an alkanoic acid and an oxygen-containing gas in the presence of the catalyst to produce an alkenyl alkanoate). Potassium acetate is preferred as the promoter and is preferably added to the extent of about 5-12 wt. %, of the weight of the reduced catalyst, preferably 6-10 wt. %, more preferably 6-8 wt. %. The promoter is preferably applied in the form of aqueous solutions. It is preferred to accomplish placement of the promoter on the reduced impregnated support beads by the xe2x80x9crotation immersion to a point of incipient wetnessxe2x80x9d technique as previously described.
The catalyst is finally dried to remove water in air or nitrogen, preferably nitrogen. During this drying, the catalyst may be heated from room temperature to a temperature of 90-150xc2x0 C., more preferably 95-120xc2x0 C. Preferably, the temperature increase from room temperature to the final drying temperature is 20-30xc2x0/minute. The catalyst is dried for at least one hour and up to 24 hours at the final drying temperature. Preferably, the catalyst is dried in nitrogen at 120xc2x0 C. for one hour.